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PEC Materials: Theory and Modeling
Database Driven Novel Photo-catalysts by Alloying
Muhammad N. Huda University of Texas at Arlington
May, 2013 Project ID # PD052
This presentation does not contain any proprietary, confidential, or otherwise restricted information
2
• Project start date: Sept, 2009 • Project end date: Dec, 2012 • Percent complete: 100% (completion of the previous project)
• Barriers addressed – Materials efficiency – Materials durability
• Total project funding $244,739 • Funding received in FY12:
$83,867 • Funding for FY13: $00
Timeline
Budget
Barriers
• Interactions/ collaborations:
• Project lead:
Partners
Overview
Approach 1: Stabilization of High Efficiency Crystalline
Material Systems
Approach 2: Enhanced Efficiency in Thin-Film Material
Systems
Effic
ienc
y
Durability
DOE Targets: >1000h @STH > 8% (2013)
Projected PEC Cost: $2 - 4/kg H2 PV-Electrolysis:
>$10/kg H2
Approach 3: Development of 3rd
Generation Materials and
Structures
Questions:
• How to discover a material with a desired set of properties?
• What would be the crystal structure for that material?
• Is that material stable? • What are electronic and optical
properties of that material?
The goal is to theoretically/computationally predict semiconductor photo-catalysts which will satisfy the following criteria to produce hydrogen by water splitting:
1 – Stable in an aqueous electrolyte
2 – The band gap in the range 1.7eV ~ 2.2eV
3 – Right band edge positions
4 – Efficient optical absorptions properties
5 – Good charge carrier transport
2 – The band gap in the range 1.7eV ~ 2.2eV
3 – Right band edge positions
Introduction to the problem:
Either band gaps are too large Or, band edges are not at the right position
Question: How do we get the right materials?
Introduction to the problem:
How do we tune band properties?
1. By isovalent doping.
2. By passive co-doping.
3. By predicting novel alloys
GaN AlxGa1-xN
GaN ZnxGa1-xN1-xOx
4. By reducing the dimensions. Nanostructures: Nanocrystal, etc.
Challenging !
•Why not doping (light or heavy)?
In doping, the overall crystal structure of the host materials remains same, but crystallinity usually deteriorate. In alloy a new crystal structure emerges, and stoichiometric relationship may change.
We first consider WO3 as our example:
Physical Review B, 77, 195102 (2008); Physical Review B, 80, 115118 (2009); Catalysis Today, 199, 53-64 (2013).
WO3
3.0
2.0
1.0
0.0
-1.0Po
tent
ial /
V v
s. N
HE H2/H2O
O2/H2O
After N-doping:
Huda et al. Physical Review B, 77, 195102 (2008). Cole et al. Journal of Physical Chemistry C, 112, 5213 (2008).
A general feature: Crystalinity is poor after doping.
To improve let us dope WO3 with nitrogen:
Mineral database search to design efficient energy-conversion materials
• There are thousands of naturally occurring minerals available, which are inherently stable.
• We plan to follow a “natural selection” process
followed by a selective band-engineering approach.
• These new materials will have better crystalline
properties than the doped materials.
Oxides those need to be avoided:
In selecting oxides we have avoided the oxides which are Mott insulators due to the following reasons:
• They have very poor transport properties for both electrons and holes.
• Doping does not improve the conduction properties significantly.
• Photo-current will be very insignificant.
Journal of Renewable and Sustainable Energy, 3, 053101 (2011).
Predicting new alloy:
Modify band structure of WO3, but not by doping.
The first step is to identify what to alloy with WO3 :
In Bi2W2O9, uplift of VB due to hybridization of O-2p and Bi-6s.
In Ag2WO4, hybridization of O-2p and Ag-4d was found favorable for water splitting.
(We do a lot of reading and testing!)
Instead of doping Ag and Bi in WO3, we search for a mineral structure which can accommodate all of these in W-oxide.
Question: What kind of atomic coordination we are looking for? What are the charge states? What symmetries? …
Need to define proper search descriptors:
Ag
AgWO4 Bi2W2O9
Goal: Instead of doping, we are looking for a mineral structure which will have both Ag and Bi in W-oxides, and a stable multi-cation oxide.
What type of bond coordination they prefer:
Mineral database Search for AgBiW2O8:
Monoclinic Orthorhombic
Derivatives
Scheelite
Possible Structures
Computation Details:
• Density functional theory (DFT) has been used to calculate the total energies and other electronic properties of the minerals.
• Vienna Ab-initio Simulation Package (VASP) was
used.
• Crystal structure relaxations were performed without any symmetry constraint.
Three most probable mineral structures for AgBiW2O9:
XRD:
Experimental (SCS and SSR) By Rajeshwar’s group at UTA
aBi-aAg-bcBiAg
aBiAg-bcBiAg
aBiAg-bcBi-bcAg
E=-186.019 eV
E= -186.009 eV
E= -185.881 eV
Inte
nsity
Wolframite structure with different layer arrangements:
2θ ChemPhysChem, 13, 2945 (2012)
Figure 5: Partial DOS and band structure of wolframite AgBiW2O8.
Second alloy structure
CuBiW2O8
Band structures
Optical Properties of CuBiW2O8:
0
20
40
60
80
100
120
0 2 4 6 8 10 12
X
Y
Z
Energy (eV)
Abs
orpt
ion
coef
ficie
nt (×
〖
〗^
〖
〗^(−
)
Third alloy structure
00.5
11.5
22.5
33.5
-10 -8 -6 -4 -2 0 2 4 6 8 10 12
W p W dO p Sn sSn p Sn dCu d
Summary
Cohesive Energy
(eV/atom) Optical Band
gap (eV)
AgBiW2O8 -2.476 2.06
CuBiW2O8 -2.547 1.48
CuSnW2O8 -3.296 0.96
CuWO4 -2.721 2.26
Collaborators: • Muhammad N. Huda (Lead Investigator, UTA) • Pranab Sarker (graduate student, UTA) • Nicolas Gaillard (HNEI) • Krishnan Rajeswar (UTA) • Todd Deutsch (NREL) • Mowafak M. Al-Jassim (NREL) • John A. Turner (NREL)
Conclusions:
We have shown that new functional materials can be designed by predicting new alloy crystal structures .
In general, multi-cation oxides (more than 2
cations) are very challenging to synthesize. This new modeling will guide the experimental
scientists by providing “what to look for” to tune the synthesize process for multi-cation oxides.
Future work:
• A robust mineral database search descriptors based on desired functionality will be developed.
• Crystal structure optimizations need to be done at a higher flexibility.
• Collaboration with synthesis groups and feedback from them will be integrated more closely.
• Nano-crystals for selected structures will be considered.